In general terms, bone cement is used for instance in connection with surgery like replacement of hip joints and spinal disc enhancements. The bone cement is a powder-like material which before use is mixed with a liquid (monomere) to form a paste. Normally, the bone cement contains acrylate plastic which is biocompatible, whereas the liquid contains an ester. For hip joint replacements, the viscosity of the bone cement paste is often quite high, whereas the bone cement paste used for spinal disc enhancements has lower viscosity.
For simplicity, the term bone cement will be used throughout this application, although the invention can be used for other kinds of paste which is used for similar surgery and which requires mixing prior to application.
An example of a prior-art device of this kind is disclosed in WO-A-04/002615 the contents of which are included herein by reference. This known device is used for mixing, dispensing and applying paste and it has been tested in practice with good results. However, there is still room for improvements.
The device of WO-A-04/002615 has a housing-cylinder assembly with a piston rod arranged displaceable and rotatable therein. At its free end within the cylinder, the piston rod is provided with mixing means and the piston rod is interconnected to a piston which is displaceable within the cylinder. In a mixing mode, the piston is kept stationary whereas the mixing means is displaced and rotated within the cylinder, by means of the rod, for mixing the bone cement and the liquid contained in the cylinder to form the aimed-at paste. In a dispensing mode, the mixing means and the piston are connected to each other to form a unit which is displaced axially within the cylinder and thereby pushes the mixed paste out of an dispensing opening at the free end of the cylinder.
In order to bring the device into dispensing mode, the operator handling the device has to turn an external ring which interconnects the piston and the piston rod so that the mixed paste can be pressed out of the cylinder. This is a somewhat difficult operation since the operator has to turn the external ring at the same time as he holds and pushes the external knob of the piston rod. Since the external ring is turnable only in one direction for connecting purposes, left-handed surgeons may find the device hard to handle in this mode.
A general requirement of devices like this, is that the displacement of the piston rod is smooth and easy to accomplish. However, the metal-against-metal contact between the piston rod and the locking members within the housing may cause scratches on the surface of the piston rod which in turn can create a risk of undesired metal fragments or particles. The scratches may also create a risk for tiny ruptures on sealing rings within the device which may lead to air leakage which in severe cases can result in undesired air cavities in the bone cement paste.